For the bars, beat with an electric mixer, in a mixing bowl, the brown sugar and oil. Add the remaining ingredients together and mix until smooth. Spray a cookie sheet with nonstick cooking spray and pour the batter into the pan. Bake at 350 for 18-22 minutes, or until the center of the bars are cooked through. While the bars are cooking, prepare the frosting.
In a sauce pan, place the butter and turn the heat to medium high. Once the butter is melted and begins to boil, watch it closely and stir constantly until you see the butter begin to turn brown. The butter will start to have a nutty, cooked smell and that is when you know to turn the butter down to medium heat. Add the brown sugar and stir continuously for 30-60 seconds, or until the brown sugar begins to dissolve. The brown sugar and butter will not incorporate very well and that is okay, you want the brown sugar to start to cook and become slightly toffee like. After the 60 seconds of boiling the brown sugar, remove the mixture from the heat and whisk in the milk, vanilla and the powdered sugar until the frosting comes together. There should be small pieces/clumps of
‘toffee’ that was created from the hot butter and brown sugar. This is what you want. Once the bars come out of the oven, spread the hot frosting over the cake as soon as possible and then top the cake with chopped pecans.